1. Field of the Invention
The present invention relates to a polymer electrolyte membrane and a fuel cell that employs the same. In particular, the present invention relates to a polymer electrolyte membrane that has excellent thermal properties and mechanical stability, and a fuel cell employing the same.
2. Description of the Related Art
A fuel cell is a device that generates electricity by a chemical reaction between the fuel and oxygen. A fuel cell can be used to provide electric power in small electronic products such as portable devices, as well as to provide electric power for industrial, household and automotive use.
Fuel cells can be classified into categories based on the type of the electrolyte to be used, including polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC), etc. The operating temperature of the fuel cell and the composition of its components vary depending on the type of electrolyte to be used.
Fuel cells can be classified based on the method of supplying fuel. These categories include an exterior reforming type that converts a fuel to a hydrogen enriched gas through a fuel reformer, a direct fuel feeding type that directly supplies a fuel in a gas or a liquid state to an anode, or an interior reforming type.
An example of the direct fuel feeding type is a direct methanol fuel cell (DMFC). In general, the DMFC uses an aqueous methanol solution as a fuel, and a hydrogen ion conducting polymer electrolyte membrane as an electrolyte. Accordingly, the DMFC is a type of PEMFC.
Although PEMFCs are small and lightweight, they can provide high output density. Furthermore, by using the PEMFC, a system for generating electricity becomes simple to construct.
A PEMFC typically comprises an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane placed between the anode and the cathode. The anode of the PEMFC is provided with a catalyst layer to promote oxidation of a fuel, and the cathode of the PEMFC is provided with a catalyst layer to promote reduction of the oxidant.
The fuel that is supplied to the anode of a PEMFC typically includes hydrogen, hydrogen-containing gas, mixed vapor of steam and methanol, and aqueous methanol solution, etc. The oxidant that is supplied to the cathode of the PEMFC typically includes oxygen, oxygen-containing gas or air.
A fuel is oxidized to form a hydrogen ion and an electron at the anode of the PEMFC. The hydrogen ion is transferred to the cathode through an electrolyte membrane, and the electron is transferred to an outer circuit (load) through a wire (or a collector). At the cathode of the PEMFC, the hydrogen ion transferred through the electrolyte membrane, the electron transferred from the outer circuit through a wire (or a collector), and oxygen are combined to form water. The flow of the electron through the anode, the outer circuit, and the cathode is electricity.
In the PEMFC, the polymer electrolyte membrane plays not only a role as an ion conductor to transfer hydrogen ions from the anode to cathode, but also a role as a separator to block the physical contact of the anode and the cathode. Accordingly, the properties required for the polymer electrolyte membrane are excellent ion conductivity, electrochemical stability, strong mechanical strength, thermal stability at operating temperature, easy thin film making, etc.
The material of the polymer electrolyte membrane generally includes a polymer electrolyte such as a sulfonate perfluorinated polymer such as Nafion® that has a backbone consisting of a fluorinated alkylene, and a side chain that consists of a fluorinated vinyl ether that has a sulfonic acid group at the terminal end. Such a polymer electrolyte membrane contains a sufficient quantity of water and thus shows excellent ion conductivity.
However, when operating a PEMFC at an operating temperature higher than 100° C., such an electrolyte membrane loses its function since its ion conductivity seriously declines due to the loss of water by evaporation. This problem makes it almost impossible to operate the PEMFC using such a polymer electrolyte membrane at atmospheric pressure and a temperature higher than 100° C. Thus, existing PEMFCs have been operated at a temperature lower than 100° C., for example at about 80° C.
Methods to increase the operating temperature of the PEMFC to a temperature of 100° C. or higher including mounting a humidifying apparatus on the PEMFC, operating the PEMFC at pressurized condition, and using a polymer electrolyte that does not require humidification have been suggested.
When the PEMFC is operated under pressurized conditions, the operating temperature can be elevated since the boiling point of water is elevated. For example, when the operating pressure of the PEMFC is 2 atm, the operating temperature can be elevated to about 120° C. However, when a pressurizing system is applied or a humidifying apparatus is mounted to the device, not only do the size and weight of the PEMFC increase, but the total efficiency of the generating system decreases. Accordingly, in order to maximize the application range of the PEMFC, the “non-humidified polymer electrolyte membrane” which is a polymer electrolyte membrane that provides excellent ion conductivity without humidification, is needed.
An example of a non-humidified polymer electrolyte membrane is disclosed in Japanese Patent Publication No. 1999-503262. In this patent, several materials, such as polybenzoimidazole, sulphuric acid or phosphoric acid doped polybenzoimidazole, etc. are described as a non-humidified polymer electrolyte.